The invention relates to methods for controlling pests in sufficiently gas-tight spaces, particularly those, in which bulk goods (agricultural products and other goods susceptible to pest attack) are stored or were stored or are to be stored loosely or packed, e.g. in bags, and other goods, e.g. of wood, by utilising gaseous phosphine (phosphorous hydride, PH3) as fumigant, as well as a device for carrying out a method of this type. In general form, the invention further relates to a process for direct production of low-concentration phosphine-air mixtures with phosphine concentrations of less than about 1.80 volume % (18.000 ppm).
It is known to rely on the biosidic properties of gaseous phosphine for controlling storage, stock and garden pests, particularly in the form of insects and rodents.
Originally, the procedure for controlling pests by fumigation of spaces in which goods, susceptible to pests attack, are stored, were stored or are to be stored, e.g. agricultural goods such as grain, utilised suitable solid metal phosphides, which release gaseous phosphine when coming into contact with moisture in the air. The metal phosphide containing preparations are, for example, introduced in the form of compressed bodies or in sachet type packaging, into which air moisture can enter and from which the gradually released phosphine can escape in very diluted form and subsequently its biosidic action can take place.
Due to various reasons, which includes the toxicity of the utilised metal phosphides, the exothermic reaction with the air moisture as well as problems of constant phosphine release in space and over time, these conventional methods exhibited however numerous disadvantages, which are further described in detail in the introductory sections of the following mentioned publications of the state of the art.
It has already been proposed, that phosphine is not applied in the form of solid forerunner compounds, but directly as gas. In this way, GB-A-2 177 004, and the corresponding South African Patent Application 864806, proposed that for fumigation a mixture of phosphine and an inert diluting gas are utilised, provided in finished form for application in pressure containers. The diluting gas is preferably carbon dioxide, and the phosphine part preferably is less than 3 volume % based on the diluting gas. This gas mixture is to be applied in such a way, that the mixture is introduced directly from the pressure container via a suitable conduit in the container or store-room, in which agricultural products, such as grains, are stored.
A considerable disadvantage with this method is that the phosphine is only provided in very diluted form, with the result that at the application point large quantities of pressure containers with the finished gas mixture have to be maintained. Furthermore, it is not guaranteed that with a simple introduction of a finished mixture of phosphine and inert gas, that the gas mixture is actually uniformly distributed in the goods to be treated and for an adequate time.
A further suggestion for fumigation by applying gaseous phosphine, is found in DE-A-37 07 453. In the mentioned disclosure, it is proposed that gaseous phosphine is discharged out of conventional steel bottles, after an inert gas, for example carbon dioxide, had previously been added. The inert gas should be added to the gaseous phosphine to prevent that ignitable phosphine-air mixtures are formed during discharge from the gas bottle. The application is to occur in such as way that after a conventional sealing of a building, large containers of a rodent construction etc., the gaseous phosphine, mixed with inert gas, is simply introduced via a pressure control/sensitive adjustment valve as well as distribution valves and hose-pipes from outside to the spaces to be fumigated (for example a building, a silo-cell, a ship-hold). It is assumed that the phosphine containing gas mixture distributes itself sufficiently in the space to be fumigated.
The method has the disadvantage, that the phosphine has to be provided as a pre-mixture with inert gas or that, besides a phosphine source in the form of a steel bottle, additionally a special inert gas source has to be provided. In any case, the method, in cases of fumigation of floor or bulk stores, for example grain storage, which are sealed from the environment by means of gas-tight folios, results in additionally added gas “inflating” the store, whereby again no uniform distribution of the phosphine in the to be treated agricultural goods is guaranteed. Additionally regarding the state of the art, reference is made to DE-A-36 08 256, DE-A-36 18 297 as well as DE-A-38 37 560, which are particularly concerned with the sealing of grain floor stores or bulk stores.
The older DE 198 39 385 A1, not disclosed before the priority date of the present invention, furthermore discloses a method, whereby in order to be able to work with concentrated phosphorous hydride, the latter is mixed with air and the resulting mixture is immediately thereafter, before discharge to the space to be fumigated, allowed to percolate through an aqueous liquid. In this manner no movement of concentrated phosphorous hydride directly into the air of the space to be fumigated occurs, because the position of the mixture production and the air space of the space to be fumigated are continuously separated by a water layer. However, under the water layer or before the discharge of the mixture in the water layer, ignitable mixtures can form. In any case, the flow speed of the air or the gas mixture is restricted, and an efficient circulation of the atmosphere of a fumigated room under enrichment with phosphorous hydride is not easily possible due to the water layer located in the flow route.